1. Field of the Invention
The present invention relates to a electric power steering device which controls a motor torque on the basis of a steering-torque signal and a speed signal in order to assist a steering, and more particularly, relates to the electric power steering device which assists a steering with a synchronous reluctance motor.
2. Description of the Related Art
A electric power steering device is a steering-force supporting device to reduce a steering force generated by a steering operation by a driver by causing an electric motor (a motor) to generate an auxiliary torque in accordance with the largeness of a steering torque of a vehicle and to transmit the auxiliary torque to a steering system. An example of such a motor used in the electric power steering device is a synchronous reluctance motor. The synchronous reluctance motor makes a rotary operation by a reluctance torque generated at a steel-plate layer of a rotator having no magnet, and in general, has a structure in which a stator has a three-phase windings, and the rotator has grooves of air (air layers) that are referred to as flux barriers provided inside the steel-plate layer. Such a structure enables the synchronous reluctance motor to generate a rotating torque (hereinafter, simply referred to as a torque) without needing an expensive rare-earth magnet or the like, resulting in a cost reduction, and good recyclability.
Also, unlike a general brushless motor having a linear torque-current characteristic, the synchronous reluctance motor has a nonlinear torque-current characteristic and calculates a target current and a current advance angle (the advancing phase angle of an armature current relative to an induced voltage) using a target torque and a motor revolutionary speed. At this time, regarding the current advance angle, an advance-angle correction value is obtained with the absolute value of the target torque being divided into three points (i.e., a region (T<T1) where the absolute value of the target torque (T) is smaller than a target torque (T1) at the maximum revolutionary speed, a region (T1<T<T3) where the absolute value of the target torque (T) is present between the foregoing target torque and a target torque (T3) that is a midpoint between a nonlinear curve of the torque-current characteristic and a linear curve thereof, and a region (T3<T) where the absolute value of the target value (T) is larger than T3), and is calculated for each revolutionary speed. The calculated target current is output in a current form of a three-phase current or a d-axis current/q-axis current.
A technology that controls a torque of the synchronous reluctance motor through such a method is disclosed in, for example, patent literature 1 and patent literature 2. The technology disclosed in these patent literatures intends to perform torque control from a low-output torque to a high-output torque by detecting the position of the rotator and the speed thereof in order to obtain a torque instruction even though the torque-current characteristic of the synchronous reluctance motor is nonlinear. In patent literature 1, an advance-angle correction value (AX) is calculated with the absolute value of a difference between a maximum revolutionary speed (N3) and a present revolutionary speed (N) being as a part of an input. Also, in patent literature 2, the advance-angle correction value (AX) is calculated from an output of a function (ADC) having the present revolutionary speed (N) as an input. That is, AX=ADC×N.    Patent Literature 1: Japan Patent No. 3209853    Patent Literature 2: Japan Patent No. 3209854
However, in the case of prior arts, conditions for obtaining the target current and the current advance angle are complicated, so that an arithmetic process time for controlling the torque of the synchronous reluctance motor becomes long, a processing capability of a CPU necessary for the arithmetic process and a memory capacity become large. Also, in the synchronous reluctance motor, the largeness of a torque and the revolutionary speed of the torque which can be output varies depending on the state of a power-source voltage, however, the technologies disclosed in both patent literatures 1 and 2 leave the variation of a power-source voltage out of consideration. Therefore, it is difficult to apply the technologies disclosed in both patent literatures 1 and 2 to the electric power steering device that uses a battery voltage because those technologies.
The present invention is made in view of these problems, and it is an object of the present invention to provide a electric power steering device that can perform good torque control through a simple arithmetic process using a synchronous reluctance motor having a nonlinear torque-current characteristic.